Enhanced oil recovery (EOR) processes employ thermal energy to facilitate the recovery of oil, particularly heavy oil, from oil-bearing geologic formations. One particular process for recovering heavy oil is referred to as steam-assisted gravity drainage (SAGD). In the SAGD process, steam is injected into the oil-bearing formation to supply thermal energy to mobilize the heavy oil. Generally, several tons of steam is required for each ton of oil recovered by the process. Injected steam heats the oil bound in the formation, and this heating lowers the viscosity of the oil. Heat from the steam comes from sensible heat as the steam cools and latent heat as the steam condenses into water. The lowered viscosity of the oil enables the oil to mix with the water, producing an oil-water mixture which may flow to collection areas and ultimately be pumped to the surface. The oil is recovered by substantially removing it from the oil-water mixture leaving a so-called produced water. The produced water is generally very warm or hot as well as being laden with contaminants in the form of suspended solids, dissolved solids, and dissolved gases. The contaminants often include compounds such as dissolved silica, hardness-producing materials, and materials giving rise to alkalinity.
It is known to process the produced water to clean the water, even if the water is to be discharged or wasted in that regulatory compliance generally requires treatment to render the water safe for discharge into surface water, for example. However, it is common to treat the produced water such that the produced water can be utilized as a feedwater to a steam generator or boiler. In the steam generator or boiler, the treated produced water is converted to steam for reuse in a SAGD process, for example. This practice is important because of the cost of water. Generating steam generator feedwater from the produced water involves substantial cleaning of the produced water to improve the quality of the water and render the water acceptable for use as feedwater for steam generation. In some cases, produced water as it is delivered from the geologic formation is too hot for optimal produced water cleaning processes so that some cooling of the produced water may be needed to remove the excess thermal energy. Excess thermal energy in produced water may be of value in pre-heating steam generator feedwater and thereby reducing the energy demand for steam generation.
Cooling of produced water and the utilization of produced water to pre-heat steam generator feedwater is typically undertaken using liquid-to-liquid heat exchange where hot produced water is directed to one side of a heat exchanger and steam generator feedwater is directed to an opposite side of the heat exchanger. Through contact with heat exchange surfaces within the heat exchanger, heat is transferred from the produced water to the steam generator feedwater. Cooled produced water and pre-heated steam generator feedwater result. However, due to the nature of typical contaminants in produced water this approach is fraught with performance problems. Contaminants form foulants in the produced water. These foulants tend to precipitate and foul the heat exchange surfaces on the produced water side of the heat exchanger resulting in degraded heat transfer effectiveness and requiring service interruptions for cleaning.
There is a need for improved methods of cooling hot produced water by transferring excess heat from produced water to pre-heat steam generator feedwater while avoiding the problems of scaling or fouling typical of liquid-to-liquid heat transfer.